As resistance to the limited number of
insecticides available for medical and veterinary pests becomes more widespread, there is an urgent need for new
insecticides and synergists on the market. To address this need, we conducted a study to assess the toxicity of three
monoterpenoids-
carvone,
menthone, and
fenchone-in comparison to
permethrin and
methomyl against adults of two common pests: the
yellow fever mosquito (Aedes aegypti) and the house fly (Musca domestica). We also examined the potential for these
monoterpenoids to enhance the effectiveness of
permethrin and
methomyl when used together. Finally, we evaluated the ability of each
monoterpenoid to inhibit
acetylcholinesterase, comparing them to
methomyl. While all three
monoterpenoids performed relatively poorly as topical
insecticides (LD50 > 4000 ng/mg on M. domestica; >6000 ng/mg on Ae. aegypti), they synergized both
permethrin and
methomyl as well as or better than
piperonyl butoxide (PBO).
Carvone and
menthone yielded synergistic co-toxicity factors (23 and 29, respectively), which were each higher than PBO at 24 h. Currently, the mechanism of action is unknown. During preliminary testing, symptoms of
acetylcholinesterase inhibition were identified, prompting further testing.
Acetylcholinesterase inhibition did not appear to explain the toxic or synergistic effects of the three
monoterpenoids, with IC50 values greater than 1 mM for all, compared to the 2.5 and 1.7 µM for
methomyl on Aedes aegypti and Musca domestica, respectively. This study provides valuable
monoterpenoid toxicity and synergism data on two pestiferous insects and highlights the potential for these chemistries in future pest control formulations.